Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 4 ■ SOCKET NAMES AND DNS
If you are curious about how this works behind the scenes, read up on the relevant international
standards starting with RFC 3492, and note that Python now includes an idna codec that can translate to
and from internationalized domain names:
>>> u'π.'.encode('idna')
'xn--hxajbheg2az3al.xn--jxalpdlp'
It is this resulting plain-ASCII string that is actually sent to the domain name service when you enter
the Greek sample domain name just shown.
Primitive Name Service Routines
Before getaddrinfo() was all the rage, programmers doing socket-level programming got by with a
simpler collection of name service routines supported by the operating system. They should be avoided
today since most of them are hardwired to speak only IPv4.
You can find their documentation in the Standard Library page on the socket module. Here, the
most efficient thing to do will be to play show-and-tell and use quick examples to illustrate each call.
Two calls let you learn about the hostname of the current machine:
>>> socket.gethostname()
'asaph'
>>> socket.getfqdn()
'asaph.rhodesmill.org'
And two more let you convert between IPv4 hostnames and IP addresses:
>>> socket.gethostbyname('cern.ch')
'137.138.144.169'
>>> socket.gethostbyaddr('137.138.144.169')
('webr8.cern.ch', [], ['137.138.144.169'])
Finally, three routines let you look up protocol numbers and ports using symbolic names known to
your operating system:
>>> socket.getprotobyname('UDP')
17
>>> socket.getservbyname('www')
80
>>> socket.getservbyport(80)
'www'
If you want to try learning the primary IP address for the machine on which your Python program is
running, you can try passing its fully qualified hostname into a gethostbyname() call, like this:
>>> socket.gethostbyname(socket.getfqdn())
'74.207.234.78'
But since either call could fail and return an address error (see the section on error handling in
Chapter 5), your code should have a backup plan in case this pair of calls fails to return a useful IP
address.
59